1. Field of the Invention
The invention relates generally to a low dropout voltage regulator, and more specifically, to a low dropout voltage regulator providing improved voltage regulation over a broad range of operating frequency.
2. Description of the Related Art
Voltage regulators can be classified into two different classes. One class is shunt regulators that place dissipative elements in parallel with a load and control the shunted current to control the output voltage. The other class is series pass regulators which places dissipative control elements between the input voltage and the load. The series pass regulators have become dominant regulators because they are significantly more efficient than the shunt regulators. The LDO voltage regulators are a type of series pass regulator that typically uses common emitter or common source output stages.
The LDO voltage regulators are voltage regulators that produce a regulated output voltage even when the unregulated input voltage from a power source falls to a level very near the regulated output voltage. The difference between the input voltage and the output voltage of the regulator is called the “dropout voltage.” In other types of voltage regulators, the dropout voltage often exceeds 2 volts. Therefore, when the power source drops below a voltage level (the regulated output voltage plus the dropout voltage), the power voltage regulators fail to deliver the regulated output voltage. The LDO voltage regulators are characterized by low dropout voltage. Therefore, the LDO voltage can provide a regulated output voltage even when other types of voltage regulators fail because of the drop in the voltage level of the power source.
FIG. 1 shows a schematic diagram of a conventional LDO voltage regulator 28. The LDO voltage regulator 28 receives an unregulated voltage VDD from a voltage input 32 and provides a regulated output voltage VO across a load RL. To achieve that result, the LDO voltage regulator 28 includes a voltage input 32 coupled to a voltage source VDD and a voltage output 30 coupled to a load RL. The LDO voltage regulator 28 also includes an error amplifier 12, a feedback path 14, a MOSFET M1, and a voltage divider comprised of two resistors R1 and R2.
A feedback voltage is obtained from the voltage divider (R1, R2) and is provided to the negative input of the error amplifier 12 through the feedback path 14. A reference voltage VREF is provided to the positive input of the error amplifier 12. An input current Iin of the LDO voltage regulator 28 is provided from a voltage source VDD to the drain of the MOSFET M1. The error amplifier 12 provides an output voltage 16 that represents a difference between the reference voltage VREF and the feedback voltage. The gate of the MOSFET M1 receives the output voltage 16 from the error amplifier 12. The source of the MOSFET M1 is coupled to the output 30 of the LDO voltage regulator 28. The MOSFET M1 provides an output voltage Vo across the load RL so that a voltage {R1/(R1+R2)×Vo} tracks the reference voltage VREF.
Conventional LDO voltage regulators do not provide desirable gain characteristics and a fast settling time over a broad range of operating frequency. This is because an LDO voltage regulator can perform only within the limits imposed by the gain-bandwidth product of the error amplifier 12. The gain-bandwidth product determines the maximum gain that can be obtained from the error amplifier 12 for a given frequency. If the error amplifier 12 is operated beyond the limits of the gain-bandwidth product, the output voltage Vo from the LDO voltage regulator 28 will be excessively distorted. Therefore, the conventional LDO voltage regulators 28 do not provide desirable gain characteristics over a wide range of the operating frequency.
Furthermore, conventional LDO voltage regulators do not provide a power saving feature. It is desirable to adjust the performance of a LDO voltage regulator based on the load condition of the output or available power from the power source. Conventional LDO voltage regulators, however, operate with the same level of performance and power consumption regardless of the load condition or power available from a power source.
Therefore, there is a need for a LDO voltage regulator that has desirable gain characteristics and response speed over a broad range of operating frequency. There is also a need for a LDO voltage regulator that has adjustable performance based on the output load condition and power available from a power source.